We need more information about your system. In particular, the type and age
of the boiler, the bypass arrangements, the type of zone valves you have and
when the system was last properly serviced.
It didn't work for me either.
Eventually I had to replace the pump and did so with one with a higher head
That seems to have alleviated the problem greatly.
I can only assume the flow rate through my conventional boiler was not high
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